Whisk egg whites in a bowl until light and foamy. Set aside.
Heat half the olive oil in a nonstick skillet over medium-low heat. Add spinach and cook until just wilted. Remove and set aside.
Wipe the skillet clean and add the remaining oil. Pour in the egg whites, spreading evenly across the pan.
Cook over low heat without stirring until almost set. Scatter spinach on one half, then gently fold the other side over.
Slide the omelette onto a plate. Season with salt, pepper, and garnish with herbs or cheese if desired.